Can I add one more chick to my 4 day old chicks?

I picked up four 2 day old chicks two days ago. My favorite had pasty butt and wouldn't eat or drink. She passed away last night. I would like to replace her. Is that possible since they are still so young ? If so, does the chick have to be the same number of days old? How long do I have before they are too old to introduce a new chick?